1. Field of the Invention
The present invention relates to a boot for a universal joint and, in particular, to a boot for a universal joint having a sealing structure that properly controls the flow of air or the like between the inside and the outside of the boot in response to a change in the inside volume of the boot based on a pressure fluctuation in the boot. The invention improves sealing performance against the intrusion of splashed water or muddy water.
2. Description of the Related Art
There has been conventionally known a boot for a universal joint having a structure which has a boot body made of an elastic material such as rubber and a boot adapter made of a plate such as a steel plate. One-end fitting portion of the boot body is fitted on the outer peripheral side of an inner shaft such as a propeller shaft or the like and the other-end fitting portion of the boot adapter is fitted on the outer race of the universal joint thereby to prevent dust or muddy water from intruding into the universal joint.
Certain features of a boot for a universal joint are already known from a boot disclosed in Japanese Patent Application Laid-open No 8-28704. The foregoing boot 010, as shown in FIG. 9A, FIG. 9B, FIG. 10A, and FIG. 10B, has a structure for coping with the pressure fluctuation in the boot 010. There, an annular lip portion 01b is in contact with the inner shaft 03 of a propeller shaft or the like outside one-end fitting portion 01a of the boot 010. A passage 01D is present for making the inside of the boot 010 communicate with the outside thereof on the inner peripheral surface of the one-end fitting portion 01a, and a sealing structure is provided with discontinuous protrusions 01c for controlling the passage 01D in response to the pressure fluctuation in the boot 010 in a circumferential direction of the inside of the annular lip portion 01b. The boot 10 controls the inflow or outflow of air or the like caused by the pressure fluctuation in the boot 010 by controlling the passage 01D via the discontinuous protrusions 01c, to reduce a load applied to the boot 010 based on the pressure fluctuation thereby to improve the durability of the boot 010.
In this respect, a part denoted by a reference symbol 0B in the FIG. 9A is a fastening band used for fitting the boot 010.
However, the boot provided with the sealing structure described above does not have sufficient measures against a detrimental effect produced by the pressure fluctuation in the boot. In particular, a pressure fluctuation may occur in the boot, for example, at a time when an abrupt pressure drop is caused in the boot by an abrupt increase in the inside volume of the boot, which is caused by the boot being expanded outward by the action of a centrifugal force produced by the rotation of the boot involved by the rotation of a shaft such as the propeller shaft or the like. In other words, the boot does not have sufficient measures against splashed water or muddy water intruding into the boot through the passage, along with the outside air being sucked into the boot when a negative pressure is produced by an abrupt pressure drop in the boot.
That is, in the sealing structure of the conventional boot 010, as is evident from FIG. 10A and FIG. 10B, when pressure in the boot 010 increases, as shown in FIG. 10A, air or the like in the boot 010 pushes up the lip portion 01b as shown by an arrow p and thus flows out. When the pressure in the boot decreases, as shown in FIG. 10B, the lip portion 01b is strongly pressed on the inner shaft 03 as shown by an arrow q to be bent inward, thereby to bring the discontinuous protrusions 01c formed in the circumferential direction on the inside of the lip portion 01b into contact with the inner shaft 03. In this way, the lip portion 01b is supported by the protrusions 01c, and hence the tip portion of the lip portion 01b is raised up in such a manner to be separated from the inner shaft 03 as shown by an arrow r. This allows the air or the like to be smoothly sucked into the boot 010 through gaps between the discontinuous protrusions 01c, as shown by an arrow s, and hence raises the possibility that splashed water and muddy water will also intrude into the boot along with the suction of the air or the like.
In particular, as described above, when negative pressure is produced by the abrupt pressure drop in the boot 010 caused by the boot 010 being expanded outward by the centrifugal force, a force for pressing the lip portion 01b in a direction shown by the arrow q is increased, to increase a tendency of raising up the tip portion of the lip portion 01b, as shown by the arrow r, thereby expanding a gap between the tip portion of the lip portion 01b and the inner shaft 03. Further, the suction force of the outside air or the like applied to the tip portion, which is produced by the negative pressure abruptly produced in the boot 010, raises the possibility that splashed water or the muddy water will intrude into the boot 010 along with the suction of the air or the like.
The foregoing conventional boot, as described above, does not have a sealing structure which copes sufficiently with the splashed water or the muddy water intruding into the boot along with the suction of the outside air. The intrusion of splashed water or muddy water into the boot causes damage to the boot itself to reduce the durability of the boot. This also deteriorates the lubrication function of grease, which is a material lubricating the inside of the boot, thereby to wear the universal joint in the boot or to form rust over a short time of use, which results in significantly reducing the durability of the universal joint.
Thus, there has been a search for a boot having the sealing structure capable of properly coping with splashed water or muddy water intruding into the boot, but the fact is that there has not been a satisfactory boot developed provided with a sealing structure having a function properly coping with pressure fluctuation in the boot, and a function properly coping with splashed water or muddy water intruding into the boot.
Therefore, it is desired to quickly develop or improve a boot provided with a sealing structure capable of properly coping with the pressure fluctuation in the boot and effectively preventing splashed water or muddy water from intruding into the boot.